The actor son of late Westies gang leader Mickey Spillane plunged to his death from his Hell’s Kitchen apartment yesterday morning as his brother, Michael, watched in horror.

Robert “Bobby” Spillane, 45, a card-carrying Teamster who had bit roles on “NYPD Blue,” “Law & Order” and “Rescue Me” and wrote an off-Broadway play about cross-dressing gangsters, fell from his sixth-floor residence on Eighth Avenue at around 8 a.m., sources said.

His uncle, James McManus, told The Post that Spillane was leaning on a window screen when it collapsed and he fell.

“He called out to his brother, and he stuck one arm out and screamed to Michael,” McManus said. “The screen then collapsed and Robert fell out with it.”

Spillane’s wife, Heather, said, “Words can’t begin to express my pain and he’ll live in our hearts forever.”

The cause is under investigation but no criminality is suspected.